The radio frame in the Long Term Evolution (LTE) system comprises frame structures in the Frequency Division Duplex (FDD) mode and Time Division Duplex (TDD) mode. The frame structure in FDD mode is shown in FIG. 1, one 10 ms radio frame is composed of 20 time slots whose length is 0.5 ms and serial numbers are 0 to 19, and the time slots 2i and 2i+1 constitutes a subframe i whose length is 1 ms. The frame structure in TDD mode is shown in FIG. 2, one 10 ms radio frame is composed of two half frames whose length is 5 ms, and one half-frame comprises 5 subframes whose length is 1 ms, and the subframe i is defined as two time slots 2i and 2i+1 whose length is 0.5 ms.
In the aforementioned two frame structures, for the Normal Cyclic Prefix (Normal CP), one time slot contains 7 symbols whose length is 66.7 microseconds (μs), the CP length of the first symbol is 5.21 μs, and the CP length of the remaining six symbols is 4.69 μs; for the extended cyclic prefix (Extended CP), one time slot contains six symbols, the CP length of all symbols is 16.67 μs. The time unit Ts is defined as Ts=1/(15000×2048) seconds, the supported uplink and downlink configuration is shown in the following Table 1, for each subframe in one radio frame, “D” denotes the subframe dedicated to the downlink transmission, “U” denotes the subframe dedicated to the uplink transmission, “S” denotes the special subframe used for three fields: the DwPTS (downlink pilot time slot), the Guard Period (GP) and the UpPTS (Uplink pilot time slot), and the lengths of the DwPTS and the UpPTS are shown in Table 2, their lengths conform to the total length of the three fields DwPTS, GP and UpPTS being 30720·Ts=1 ms. Each subframe i is represented by the two time slots 2i and 2i+1, and the length of each time slot is Tslot=15360·Ts=0.5 ms.
The LTE TDD supports the 5 ms and 10 ms uplink and downlink switching cycles. If the downlink to uplink transition point cycle is 5 ms, the special subframe exists in two half frames; if the downlink to uplink transition point cycle is 10 ms, the special subframe exists only in the first half frame. The subframe 0, the subframe 5 and the DwPTS are always used for downlink transmission. The UpPTS and the subframe next to the special subframe are dedicated to the uplink transmission.
TABLE 1UL/DL configurationDownlink-UL/DLuplinkconfig-transition pointSubframe numberurationcycle012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
TABLE 2Special subframe configuration (DwPTS/GP/UpPTS length)Normal cyclic prefix, downlinkExtended cyclic prefix, downlinkUpPTSUpPTSNormalNormalSpecialcyclicExtendedcyclicExtendedsubframeprefix,cyclic prefix,prefix,cyclic prefix,configurationDwPTSuplinkuplinkDwPTSuplinkuplink0 6592 · Ts2192 · Ts2560 · Ts 7680 · Ts2192 · Ts2560 · Ts119760 · Ts20480 · Ts221952 · Ts23040 · Ts324144 · Ts25600 · Ts426336 · Ts 7680 · Ts4384 · Ts5120 · Ts5 6592 · Ts4384 · Ts5120 · Ts20480 · Ts619760 · Ts23040 · Ts721952 · Ts———824144 · Ts———
In the LTE, the physical downlink control channel (PDCCH) is used to carry the uplink and downlink scheduling information as well as the uplink power control information. The downlink control information (referred to as DCI) format is classified as the DCI formats 0, 1, 1A, 1B, 1C, 1D, 2, 2A, 3, 3A and so on. The e-Node-B (referred to as eNB) can configure the User Equipment (referred to as UE) through the downlink control information, or the user equipment accepts the configuration of higher layers, also known as configuring the UE through higher-layer signaling.
The SRS (Sounding Reference Signal) is a signal used to measure the Channel State Information (referred to as CSI) between the user equipment and the eNB. In the Long Term Evolution system, the UE regularly sends the uplink SRS in the last data symbol of the transmitted subframe in accordance with the bandwidth, the frequency domain location, the sequence cyclic shift, the period, the subframe offset and other parameters indicated by the eNB. The eNB judges the uplink CSI of UE according to the received SRS and performs the frequency domain selection scheduling, the closed-loop power control and other operations according to the obtained CSI.
In the LTE system, the SRS sequence sent by the UE is obtained by performing cyclic shift α on one root sequence ru,v(n) in the time domain. Different SRS sequences can be obtained by performing different cyclic shifts a on the same root sequence, and the obtained SRS sequences are orthogonal to each other, therefore, these SRS sequences can be allocated to different UEs to use, so as to achieve the CDMA between UEs. In the LTE system, the SRS sequence defines eight cyclic shifts a, given by the following formula (1):
                    α        =                  2          ⁢                                          ⁢          π          ⁢                                    n              SRS              cs                        8                                              Equation        ⁢                                  ⁢                  (          1          )                    
wherein, nSRScs is indicated by a 3 bit signaling, 0, 1, 2, 3, 4, 5, 6 and 7 respectively. In other words, in the same time-frequency resource, the UEs in the cell have 8 available code resources, and the eNB can configure up to eight UEs to send the SRS simultaneously in the same time-frequency resource. The equation (1) can be regarded as dividing the SRS sequence into eight portions with the same interval in the time domain, but since the length of the SRS sequence is a multiple of 12, the minimum length of the SRS sequence is 24.
In the LTE system, the SRS frequency domain bandwidth is configured with the tree structure. Each SRS bandwidth configuration corresponds to one tree structure, and the highest layer (or the first layer) SRS-Bandwidth corresponds to the maximum SRS bandwidth of that SRS bandwidth configuration, or called as the SRS bandwidth range. According to the eNB's signaling instruction, after the UE calculates to obtain its own SRS bandwidth, it determines the frequency domain initial position where the UE sends the SRS according to the upper-layer signaling frequency domain position nRRC sent by the eNB. FIG. 3 is a schematic diagram of the frequency domain initial position for the UEs with different allocated nRRC to send the SRS in the related art, as shown in FIG. 3, the UEs with different allocated nRRC sends the SRS in different regions of the cell SRS bandwidth, wherein the UE1 determines the frequency initial position of sending the SRS according to nRRC=0, the UE2 determines the frequency initial position of sending the SRS according to nRRC=3, the UE3 determines the frequency initial position of sending the SRS according to nRRC=4, and the UE4 determines the frequency initial position of sending the SRS according to nRRC=6.
The sequence used by the SRS is selected from the demodulation pilot sequence group, and when the SRS bandwidth of the UE has 4 Resource Blocks (referred to as RB), a Computer Generated (referred to as CG) sequence whose length is 2 RBs is used; when the SRS bandwidth of the UE is greater than 4 RBs, the Zadoff-Chu sequence with the corresponding length is used.
In addition, in the same SRS bandwidth, the SRS sub-carriers are placed with a certain interval, that is, the SRS transmission uses the comb structure, and the number of frequency combs in the LTE system is 2, and the corresponding Repetition Factor (referred to as RPF) in the time domain is 2. FIG. 4 is a schematic diagram of the SRS comb structure in the related art, and as shown in FIG. 4, when each UE sends the SRS, it uses only one of the two frequency combs, comb=0 or comb=1. Therefore, the UE only uses the sub-carriers whose frequency domain indexes are even or odd to send the SRS according to the instruction of the 1-bit upper-layer signaling. This comb-like structure allows more UEs to send the SRS in the same SRS bandwidth.
In the same SRS bandwidth, multiple UEs use different cyclic shifts in the same frequency comb, and then send the SRS through the code division multiplexing, or two UEs send the SRS through the frequency division multiplexing in the different frequency comb. For example, in the LTE system, there are 8 cyclic shifts and two frequency combs which can be used by the UEs to send the SRS in a certain SRS bandwidth (4 RBs), therefore, the UEs have 16 resources for sending the SRS in total, that is, in this SRS bandwidth, up to 16 SRSs can be sent at the same time. Since the uplink Single User Multiple Input Multiple Output (referred to as the SU-MI MO) is not supported in the LTE system, the UE can only have one antenna to send the SRS at each moment, therefore, one UE only requires one SRS resource, therefore in the aforementioned SRS bandwidth, the system can multiplex up to 16 UEs simultaneously.
The LTE-Advanced (referred to as LTE-A) system is a next generation evolved system of the LTE system, and it supports the SU-MIMO in the uplink and can use up to four antennas as the uplink transmitting antennas. That is, the UE can send the SRS with multiple antennas at the same time, and the eNB needs to estimate the state of each channel based on the SRS received by each antenna.
In the existing LTE-A research, it proposes that in the uplink communication, it should use the non-precoded (that is, antenna dedicated) SRS. At this point, when the UE uses multiple antennas to send the non-precoded SRS, the SRS resources needed by each UE will increase, causing the number of UEs that can be multiplexed in the system to decrease at the same time. In addition, besides of reserving the LTE's original SRS which is sent periodically, the downlink control information or the higher-layer signaling can also be used to configure the UE to send the SRS aperiodically.
For example, in a certain SRS bandwidth (4 RBs), if every UE uses four antennas to send the SRS, the number of resources needed by each UE is 4. The total number of SRS resources supported in one SRS bandwidth is 16, and then in this SRS bandwidth, the number of UEs that can be multiplexed is reduced to four. In the system, the number of users that can be multiplexed at the same time is ¼ of that in the original LTE.
Also, since it is proposed in the requirement of LTE-A that the number of users accommodated in the LTE-A system should not be less than that in the LTE system, thus this requirement contradicts the fact that the number of users decreases when sending the SRS with multiple antennas.